It is already known to use a catalyst system comprising a transition metal compound and an organometallic compound for low-pressure polymerization of olefins. In recent years, a large number of proposals have been made for producing, as a highly active catalyst, a solid catalyst component comprising magnesium, titanium and halogen as main components.
However, to obtain higher productivity, it is very important from the industrial viewpoint to aim not only at high catalytic activity but also at improving the particle size, the bulk density and the like of polymer particles. At the same time, it is now demanded to optionally control the molecular weight distribution of the resulting polymer for satisfying the diversification of qualities.
In Japanese Examined Patent Publication No. 39714/1977, the present inventors previously proposed a polymerization method which can produce a polyolefin having a desired molecular weight distribution while maintaining high catalytic activities by employing a catalyst system comprising a reaction product of metal magnesium, a hydroxylated organic compound, an organic oxidized compound of a transition metal, a halogen-containing compound of a transition metal and an aluminum halide, and an organometallic compound, but it was still insufficient from the viewpoint of improvement in the particle properties such as the particle size, bulk density and the like of polymer particles.
A serious problem which usually occurs in conducting polymerization is soiling of a reactor (deposition of a solid polymer on the inner surface of the reactor), whereby it is necessary to stop the polymerization operation to remove the deposition. Such a problem tends to be significant particularly in the production of a resin having a broad molecular weight distribution such as the one which is to be used for blow molding, extrusion molding or the like.
On the other hand, if the particle size distribution is broad, various problems are likely to occur during the production of a polyolefin, for example, in the steps of polymerization, separation of the particles from the polymer slurry, drying of the powder and transportation of the powder, and in some cases, long-term continuous production becomes impossible. Further, when a polymer is obtained by multi-stage polymerization, if the particle size distribution of the polymer particles is broad, classification of the powder is likely to occur after the drying step, i.e. at the stage where additives are added or at the stage of transportation. The physical properties differ from one particle size to another and the adverse effects to the quality can sometimes not be ignored. Further, in a gas phase polymerization, particle properties are particularly important in view of productivity because, if the powder properties are poor, blocking or the like of polymer particles occurs in the polymerization reactor.
Under these circumstances, in Japanese Examined Patent Publication No. 48682/1987, the present inventors previously proposed a method for improving or eliminating the above defects. In this method, a catalyst system is employed which comprises a solid catalyst component (A) obtained by reacting metal magnesium and a hydroxylated organic compound, or an oxygen-containing organic compound or halogen-containing compound of magnesium, with an oxygen-containing organic compound or halogen-containing compound of titanium, an oxygen-containing organic compound or halogen-containing compound of zirconium, a silicon compound, and an aluminum halide compound, and a catalyst component (B) of an organometallic compound.
However, this method could not lead to an improvement in the particle size distribution.